Position determining systems are useful both for those who need to know their own location and for those who need to know the locations of others. One system, the Loran system, was developed in the United States during World War II and was maintained by the U.S. Coast Guard as an aid to ships and aircraft. The smallest working component of the Loran system requires two transmitters, a master and a slave repeating the master, and a receiver. The receiver tracks the master and slave signals with phase-locked loops and presents their time differences on a display. For each time difference there is a hyperbolic line of position. Accordingly, systems of this type are called hyperbolic systems. The time differences can be transferred to a map on which hyperbolic lines of position have been printed. Alternatively, a computer can provide direct readouts in latitude and longitude. The Loran network, however, does not cover major portions of the globe and its receivers are generally expensive.
The Global Positioning System (GPS) is a network of approximately 24 satellites and a dozen ground stations. The GPS network currently provides navigation information worldwide. A receiver derives its three-dimensional position from ranging signals received from three or more satellites. The Loran and GPS networks, however, do not provide any signal relays in a crisis. Thus, if one had a GPS receiver, the position of an emergency would be known, but there would be no way within the system to communicate that position to potential rescue teams.
Emergency radio beacons are often used in conjunction with satellites to indicate an accident or crisis. Beacons of this type typically transmit a signal which is received by a satellite which then retransmits the signal to an earth station for computer analysis. The computer provides a position estimate, and search aircraft then use conventional techniques to locate the radio beacon. The initial location is usually calculated using doppler shift information at the earth station.
For example, U.S. Pat. No. 4,240,079 to Zhilin discloses a location system in which a mobile emergency radio beacon can be manually or automatically activated. A coded signal from the emergency radio beacon is received by a satellite and retransmitted to a receiving station directly or via a geostationary satellite. The receiving station then calculates the position of the emergency radio beacon. Geostationary satellites, however, require fairly large, high-powered, and expensive terminals on the ground. High powered terminals on the ground are required because of the long transmission distances from earth and the need to use microwaves (L-Band and above) to form narrow beams on the satellites.
Another problem with emergency beacons is that they usually transmit their homing signal continuously. A standard emergency beacon continuously transmits for two basic reasons. First, continuous radiation provides a base station with a reference from which to calculate an initial approximate position. Second, the continuous radiation acts as a homing beacon which radiates until a final rescue. However, if a rescue team takes a number of hours to arrive at a remote location, then the battery of a conventional emergency beacon could expire. If a continuously transmitting emergency beacon depletes its battery supply prior to a rescue team's arrival, then the rescue team must resort to traditional and less precise search methods. Moreover, those in need of remote rescue do not usually remain conveniently in place near an initial approximate position. Thus, an individual adrift in a lifeboat at sea is subject to winds and currents, and those lost in a forest may attempt to hike out. In these cases, an emergency beacon which runs out of battery power can offer an initial distress call, but the approximate position determined by base station computers provides only a reference point for search and rescue teams.
A global messaging network, known as ORBCOMM, has been proposed by the Orbital Communications Corporation of Fairfax, Va. The ORBCOMM system is described in a brochure published by the Orbital Communications Corporation entitled ORBCOMM, Vital Communications Absolutely Anyplace on Earth. The ORBCOMM system is designed to bring data communications and position determination to a multitude of mobile remote units. The ORBCOMM network provides low speed VHF digital data communications using low earth orbiting satellites that have a very high availability. As shown in FIG. 1, the basic ORBCOMM network uses a network of satellites 111, a network control station 112, and a base station 113 to communicate with the remote units 110. A typical message of around 100 bytes or characters will be transmitted from a remote unit 110 through the satellite 111 and to a network control center 112. The data can then be stored at the network control center 112 and accessed at a customer's convenience, similar to electronic mail. A customer at a base station 113, which could be a portable computer with a MODEM, can then poll the network control center 112 for messages via a standard phone line 117.
The remote units 110 contemplated by ORBCOMM do offer some emergency features. For example, an emergency alerting option can send a short emergency alert message to a customer's base station 113 upon user activation. The remote unit 110 continues to send the alert until it receives confirmation of its receipt. The remote unit 110 can also determine and send its approximate position with the emergency alert. The ORBCOMM network, however, does not provide a mechanism by which a position determining team can home in on the individual in distress. Consequently, if an individual in distress transmits an emergency alert with an approximate position that is more than a kilometer distant from his location, then the search and rescue team must still resort to traditional search methods after they reach the approximate position.